A potentiometer of this kind is disclosed in U.S. Pat. No. 4,433,287 at FIG. 6 thereof, where a fixed resistor Rf having a resistance of 100r, a variable resistor Rv, and an impedance transformer A1 are connected. The variable resistor Rv comprises a series combination R1 of resistors each having a high resistance of 100r, a series combination R2 of resistors each having an intermediate resistance of 10r, and a series combination R3 of resistors each having a low resistance of r. When sliders are placed in arbitrary positions along the resistor groups R1-R3 having such different orders of magnitude of resistances, i.e., when X resistors of the group R1, Y resistors of the group R2, and Z resistors of the group R3 are connected into the circuitry, the input voltage Vi is given by the relationship ##EQU1## where V.sub.0 is the output voltage.
Thus, the output voltage gives a full-scale indication while the input voltage provides an indication magnified according to the multiplier expressed by three digits X, Y, Z and is measured. For example, at the illustrated positions, V=3.62 V.sub.0.
When this potentiometer is employed, the full-scale reading of the voltage-measuring instrument can be widely varied at will in minute steps. Further, the set value can be read directly as the measured value. However, the potentiometer is bulky, because as many resistors as the steps of resistance are needed for each order of magnitude of resistance, as well as the associated switching circuit. Further, the use of a rotary switch makes the potentiometer unsuitable for digital control.
In consideration of the foregoing, another potentiometer for a voltage-measuring instrument has been proposed in Japanese patent application No. 262983/1985, where banks of resistors are connected in series according to the number of digits of the set multiplier to form a variable resistor. Each bank of resistors consists of four resistors whose resistances are respectively 1, 2, 4, 8 times a minimum resistance. A digital switch or the like produces a BCD (binary-coded decimal) signal to control the multiplier. Yet, this potentiometer does not allow the multiplier to be set less than the greatest order of magnitude of resistance. Therefore, a maximum voltage division ratio greater than 10:1 cannot be obtained.